Cardiovascular System Block Cardiac Arrhythmias (Physiology )

1. Cardiovascular System BlockCardiac Arrhythmias(Physiology) Dr. Mona Soliman, MBBS, MSc, PhD Associate Professor Department of Physiology Chair of Cardiovascular Block College of Medicine King Saud University

2. Lecture Objectives • Describe sinus arrhythmias • Describe the main pathophysiological causes of cardiac arrhythmias • Explain the mechanism of cardiac block • Explain the origin of an ectopic foci • Enumerate the common arrhythmias and describe the basic ECG changes

3. Normal Sinus Rhythm • Regular • Single p-wave precedes every QRS complex • P-R interval is constant and within normal range • P-P interval is constant

4. Causes of Cardiac Arrhythmias • Abnormal rhythmicity of the pacemaker • Shift of the pacemaker from the sinus node to another place in the heart • Blocks at different points during the spread of the impulse through the heart • Abnormal pathways of impulse transmission through the heart • Spontaneous generation of spurious impulses in almost any part of the heart

5. Causes of Cardiac Arrhythmias • Rate above or below normal • Regular or irregular rhythm • Narrow or broad QRS complex • Relation to P waves

6. Abnormal Sinus Rhythm • Tachycardia: an increase in the heart rate • Heart rate > 100 beats per minute • Causes: • Increased body temperature • Sympathetic stimulation • Drugs: digitalis • Inspiration

7. Abnormal Sinus Rhythm • Bradycardia: • Slow heart rate < 60 beats per minute • Causes: • Parasympathetic stimulation • Expiration

8. Abnormal Cardiac Rhythms that Result from Impulse Conduction Block • Sinoatrial Block • Blockasde of the S-A node impulse before entering atrial muscle • Cessation of P wave • Causes: • Ischemia of the A-V node • Compression of the A-V node by scar formation • Inflammation of the A-V node • Strong vagal stimulation

9. Abnormal Cardiac Rhythms that Result from Impulse Conduction Block • A-V Block • When impulse from the S-A node is blocked • Causes: • Ischemia of the A-V node • Compression of the A-V node by scar formation • Inflammation of the A-V node • Strong vagal stimulation

10. Types of the A-V Block • First degree block • Second degree block • Third degree block

11. Types of the A-V Block • First degree block • Prolong P-R interval (0.2 seconds)

12. Types of the A-V block Second Degree Block • P-R interval > 0.25 second • Only few impulses pass to the ventricles  atria beat faster than ventricles “dropped beat” of the ventricles

13. Types of the A-V block Third degree block (complete) • Complete dissociation of P wave and QRS waves • Ventricle escape from the influence of S-A node • Atrial rate is 100 beats/min • Ventricular rate is 40 beats/min • Stokes-Adams Syndrome: AV block comes and goes

14. Premature contractions • Premature contractions, extrasystoles, or ectopic beat result from ectopic foci that generate abnormal cardiac impulses (pulse deficit) • Causes: • Ischemia • Irritation of cardiac muscle by calcified foci • Drugs like caffeine • Ectopic foci can cause premature contractions that originate in: • The atria • A-V junction • The ventricles

15. Premature Atrial Contractions • Short P-R interval depending on how far the ectopic foci from the AV node • Pulse deficit if there is no time for the ventricles to fill with blood • The time between the premature contraction and the succeeding beat is increased (Compensatory pause)

16. Premature Ventricular Contractions (PVCs) • Prolong QRS complex because the impulses are carried out with myocardial fibers with slower conduction rate than Purkinje fibers • Increase QRS complexes voltage because QRS wave from one ventricle can not neutralize the one from the other ventricle • After PVCs, the T wave has an electrical potential of opposite polarity of that of the QRS because of the slow conduction in the myocardial fibers, the fibers that depolarizes first will repolarize first • Causes: drugs, caffeine, smoking, lack of sleep, emotional irritations

17. Ventricular Fibrillation • The most serious of all arhythmias • Cause: impulses stimulate one part of the ventricles, then another, then itself. Many part contracts at the same time while other parts relax (Circus movement)

18. Ventricular Fibrillation • Causes: sudden electrical shock, ischemia • Tachycardia • Irregular rhythm • Broad QRS complex • No P wave

19. Ventricular Fibrillation • Treatment : DC shock

20. Atrial Fibrillation • Same mechanism as ventricular fibrillation. It can occur only in atria without affecting the ventricles • It occurs more frequently in patients with enlarged heart • The atria do not pump if they are fibrillating • The efficiency of ventricular filling is decreased 20 to 30% • No P wave, or high frequency of low voltage P wave • Treatment: DC shock

21. Atrial Flutter • A single large wave travels around and around in the atria • The atria contracts at high rate (250 beats/min) • Because one area of the atria is contracted and another one is relaxed, the amount of blood pumped by the atria is slight • The refractory period of the AV node causes 2-3 beats of atria for one single ventricular beat 2:1 or 2:3 rhythm

22. Ischemia and the ECG • One of the common uses of the ECG is in acute assessment of chest pain • Cause: restriction of blood flow to the myocardium, either: • Reversible: angina pectoris • Irreversible: myocardial infarction • Ischemia  injury  infarction

23. Reversible ischemia • Inverted T wave • ST segment depression

24. Myocardial Infarction • Complete loss of blood supply to the myocardium resulting in necrosis or death of tissue • ST segment elevation • Deep Q wave

25. Potassium and the ECG • Hypokalemia: • flat T wave • Hyperkalemia: • Tall peaked T wave

26. For further readings and diagrams: Textbook of Medical Physiology by Guyton & Hall Chapter 10 (Cardiac Arrhythmias and their Electrocardiographic Interpretation)